A few that I have saved on my computer
http://www.columnar-cacti.org/
http://www.mammillarias.net/gallery/
If you like Copiapoa's here are a couple the first one has incredible photos
http://www.gargamel-cactus.com/photos/Chili%2C-Atacama%2C-Copiapoa
http://www.copiapoa.info/
Illustated lexicon of cactus and succulents
http://www.astrokaktus.com/CactusBase/Vstop/a_cactusbase-en.html
Cacti database
http://www.thater.net/cactaceae/db/
Thousands of photo to help with ID's
http://www.geocities.com/anvitel/a_e.htm